Method of introducing residual compressive stresses into a shaft

ABSTRACT

The invention relates to a method of introducing compressive residual stresses into shaft notches of a shaft which is configured as a stepped shaft having successive stages having a different diameter. Diameter transitions or notch regions are located between each two adjacent stages. The diameter transitions or notch regions are quenched in a controlled manner as part of a heat treatment of the shaft.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2007/051744, filed Feb. 23, 2007 and claims the benefitthereof. The International Application claims the benefits of Europeanapplication No. 06011627.4, filed Jun. 6, 2006, both of the applicationsare incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to a method of introducing residual compressivestresses into shaft notches of a shaft which is configured as a steppedshaft having successive stages of a different diameter, wherein diametertransitions or notch regions are arranged between respectively adjacentstages.

BACKGROUND OF THE INVENTION

Shafts of this type are known and are used, for example, inturbomachines, for example steam turbines, which have a low-pressurepart and can therefore also be referred to as low-pressure shafts. Theshaft bears rotor blades which, together with associated guide vanes,form a blade cascade through which a flow medium, for example steam,flows.

Particularly in the low-pressure part of the turbomachine, the shaftconsists of a base material which is tough at low temperatures; by wayof example, 2-3.5-NiCrMoV steels are used for producing the low-pressureshaft.

The flow medium partly acts as a corrosive medium on the components ofthe turbomachine, for example on disks in disk rotors or on regions ofthe shaft, particularly of the shafts of low-pressure subturbines, whichare close to the surface. The influence of these corrosive media canconsiderably reduce the fatigue strength of the base material. However,a reduction in the fatigue strength of the base material, for example ofthe shafts in low-pressure subturbines, also disadvantageously reducesthe service life of the shaft.

In order to solve this problem, it is known to carry out fatiguestrength tests under the influence of corrosive media, with appropriatedesign data being provided for use in the calculation (lowered withrespect to ambient air). However, it is also known to reduce operatingstresses by introducing, for example, residual compressive stresses intolow-pressure drivers and groove regions by means of roller-burnishing orshot-peening in the finish-machined state or given the ultimate finalcontour of the shaft. However, it is also possible to introducecompressive stresses in the notch-free region of the shaft by means of asuitable heat treatment. During the production of the shafts, it isnecessary to maintain the strictest tolerances, it being possible forthe service life of the components, particularly of the shafts, to bereduced by cracks emanating from existing diameter transitions or notchregions. The crack sensitivity, particularly at diameter transitions ornotch regions, affects the service life of the shafts extremelydisadvantageously (component failure).

SUMMARY OF THE INVENTION

The invention is based on the object of improving a method ofintroducing residual compressive stresses into shaft notches of a shaftof the type mentioned in the introduction using simple means, to theeffect that the resistance to component failure as a result of corrosionand dynamic loading is considerably improved.

According to the invention, the object is achieved by virtue of the factthat the diameter transitions or notch regions of the shaft are quenchedin a controlled manner after a final tempering treatment, for example ahardening and tempering heat treatment, at tempering temperature and/orbelow the tempering temperature.

The shaft, particularly the shaft notches thereof, is thereby protectedagainst a reduction in fatigue strength as a result of, for example, wetsteam. In this case, in addition to the application of a protectivelayer, for example, a method of specifically increasing the residualcompressive stresses in diameter transitions or notch regions isadvantageously carried out according to the invention.

It is advantageously provided that the diameter transitions or notchregions are specifically sprayed with a cooling liquid or a quenchingmedium for quenching purposes. However, for the purposes of controlledquenching, it may also be provided that the shaft as a whole istransferred into a dipping bath.

It is also possible for the tempering treatment of the hardening andtempering to be followed by a separate heat treatment which has the soleaim of introducing residual compressive stresses. In order to avoidinfluencing the mechanical properties achieved, this expedientlyinvolves selecting a temperature which is sufficiently different fromthe final heat-treatment temperature but which is still high enough toachieve the desired effect.

It is possible to select any suitable medium, preferably water, as thecooling liquid or quenching medium for quenching purposes; however, itis also possible to use air/water mixtures, suitable polymers or oil andemulsions as the cooling liquid or quenching medium.

In order to ensure that a possible distortion of the component or of theshaft can be compensated for after the quenching (spraying or dipping),it is advantageous within the context of the invention if the diametertransitions or the notch regions in a heat-treatment contour areproduced with an allowance provided in relation to an ultimate finalcontour, wherein the heat-treatment contour is removed during theproduction of the ultimate final contour after the quenching. After thefinish-machining or the production of the ultimate final contour, theprovided allowance means that sufficiently high residual compressivestresses are maintained in the shaft surface, specifically in thetransition radii (diameter transitions or notch regions), with a definedaction at depth.

It is expedient within the context of the invention if the allowance hasa magnitude of at most from 10 to 40 mm on final use in relation to thefinal contour of the shaft.

It is advantageously provided that the heat-treatment contour of thediameter transitions or notch regions has a radius with a magnitude R offrom 25 to 50 mm. The radii (diameter transitions or notch regions) ofthe heat-treatment contour of the shaft are accordingly configuredspecifically with a defined dimension as a function of the residualcompressive stresses and depth distribution required in the finalcontour.

Overall, the method according to the invention reduces the local stressload during shutdown and during operation of the shaft. In addition,crack sensitivity is reduced at radii or diameter transitions, and thisleads to an improved or increased service life of the shaft or of thecomponent treated in accordance with the invention. As a result ofspecifically setting residual compressive stresses of from −100 to −400MPa at the shaft surface, particularly in the diameter transitions ortransition radii as a result of the specific quenching of the shaft,even relatively large defects which are close to the surface of thetreated component or of the shaft may be admissible, and therefore theshaft as a whole can be produced at lower cost since stricter tolerancesfor defects which may arise from the production process no longernecessarily have to be maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantageous refinements of the invention are disclosed in thedependent claims and the description of the FIGURE below:

FIG. 1, the only FIGURE, shows a basic illustration of a shaft for alow-pressure part of a turbomachine or of a steam turbine.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a detail of a shaft 1 which is configured as a steppedshaft having successive stages 2 of a different diameter D1 to D4 inrelation to a mid-axis X, with four stages 2 being illustrated by way ofexample. The exemplary shaft 1 illustrated is a component of alow-pressure part of a turbomachine, for example of a low-pressuresubturbine of a steam turbine. The shaft 1 is produced, for example,from a material which is tough at low temperatures; for example,2-3.5-NiCrMoV steels are used for producing the low-pressure shaft.However, it is of course also possible for the shafts to be producedfrom other materials or material combinations.

Diameter transitions 3 or notch regions are arranged between each twoadjacent stages 2. The diameter transitions 3 are designed in relationto the mid-axis X to be slightly curved or convex with a radius R in thedirection toward the mid-axis X.

In order to achieve a specific setting of residual compressive stressesof from −100 to −400 MPa at the shaft surface, particularly at thediameter transitions 3 or the transition radii, said diametertransitions or the notch regions are quenched in a controlled manner aspart of a heat treatment or after heating of the shaft.

The diameter transitions 3 or notch regions are preferably quenched in acontrolled manner after a final tempering treatment at temperingtemperature. A subsequent, separate heating and quenching after thetempering as a separate process step is of course also possible.

In the exemplary embodiment illustrated in FIG. 1, a cooling liquid or aquenching medium is sprayed onto the diameter transitions 3 for thepurpose of controlled quenching after the final tempering treatment attempering temperature, and this is illustrated by means of thefan-shaped spray jets 4. It is possible to select any suitable medium,preferably water, as the cooling liquid or quenching medium forquenching purposes; however, it is also possible to use air/watermixtures, suitable polymers or oil and emulsions. The shaft 1 canhowever also be dipped as a whole.

In the exemplary embodiment illustrated in principle in FIG. 1, theshaft 1 is configured with a heat-treatment contour 6. In relation to afinal contour (dashed line 7, illustrated in exaggerated fashion forillustration purposes), that is to say of a finish-machined shaft 1 withits final contour finish-machined for installation in the low-pressurepart of the steam turbine, the heat-treatment contour 6 has an allowance8 of at most from 10 to 40 mm in the respective shaft radius r1 to r4(r=D/2).

The allowance 8 of the shaft 1 in the heat-treatment contour 6 istherefore specifically increased for the hardening and tempering heattreatment (tempering treatment) by at most 10 to 40 mm, preferably inthe respective shaft radius r or the respective diameter transition 3,with respect to the final end contour 7. This ensures that a possibledistortion of the shaft 1 can still be compensated for after thequenching (spraying or dipping).

It is also possible to carry out machining, for example in the case ofdouble tempering treatment. In this case, the allowances can then beadapted or machining may also be effected when a separate heat treatmentfor producing residual (compressive) stresses is carried out after thetempering treatment from the hardening and tempering.

After the finish-machining (production of the ultimate final contour),this maximum allowance 8 means that sufficiently high residualcompressive stresses are maintained in the shaft surface andspecifically in the transition radii or diameter transitions 3, with adefined action at depth. In the heat-treatment contour 6, the radii Rhave a magnitude of R of approximately equal to 25 to 50 mm.

The respective transitions from the diameter transitions 3 to therespective stages 2 are illustrated in exaggerated fashion in theexemplary embodiment and are, of course, correspondingly machined atleast for producing the ultimate final contour.

1-6. (canceled)
 7. A method for introducing a residual compressivestress into a shaft having successive stages of different diameters,comprising: arranging a diameter transition between adjacent stages ofthe successive stages; and controlled quenching the diameter transitionafter a final tempering treatment as part of a hardening and temperingheat treatment.
 8. The method as claimed in claim 7, wherein thediameter transition is sprayed with a quenching medium for thequenching.
 9. The method as claimed in claim 7, wherein the diametertransition is cooled by a dipping operation for the quenching.
 10. Themethod as claimed in claim 7, wherein the diameter transition in aheat-treatment contour of the shaft is produced with an allowance to anultimate final contour of the shaft.
 11. The method as claimed in claim10, wherein the heat-treatment contour is removed during producing theultimate final contour of the shaft or after the quenching.
 12. Themethod as claimed in claim 10, wherein the allowance is in a magnitudeof at most 10 to 40 mm.
 13. The method as claimed in claim 11, whereinthe heat-treatment contour of the diameter transition has a radius witha magnitude of 25 to 50 mm.
 14. The method as claimed in claim 7,wherein the residual compressive stress is introduced into a shaft notchof the shaft.
 15. The method as claimed in claim 7, wherein the shaft isused in a turbomachine.
 16. A method for introducing a residualcompressive stress into a shaft notch of a shaft having successivestages of different diameters, comprising: arranging a notch regionbetween adjacent stages of the successive stages; and controlledquenching the notch region after a final tempering treatment as part ofa hardening and tempering heat treatment.
 17. A turbomachine,comprising: a shaft comprising: successive stages of differentdiameters; a diameter transition arranged between adjacent stages of thesuccessive stages that is controlled quenched after a final temperingtreatment as part of a hardening and tempering heat treatment.